Photography, since its emergence as a practical art in the last half of the ninteenth century, has relied upon elements containing radiation-sensitive silver halide to serve a wide range of imaging needs. As compared to available imaging alternatives, silver halide photographic elements exhibit a combination of very advantageous properties, including higher speed and better image definition. Further, silver halide photographic elements are virtually unique in their highly refined capability for accurately reproducing multicolor images.
Over the last century silver halide photographic elements have retained their prominent position in the photographic industry by reason of intensive and painstaking investigations, both theoretically and empirically based, aimed at better understanding and improving photographic capabilities. Extensive academic and industrial research has been devoted to the improvement of silver halide photographic elements, and thousands of patents have been issued, attesting to the vigor with which improvement has been pursued.
a. Speed, granularity, and sensitization
Silver halide photography employs radiation-sensitive emulsions comprised of a dispersing medium, typically gelatin, containing embedded microcrystals--known as grains--of radiation-sensitive silver halide. During imagewise exposure a latent image center, rendering an entire grain selectively developable, can be produced by absorption of only a few quanta of radiation, and it is this capability that imparts to silver halide photography exceptional speed capabilities as compared to many alternative imaging approaches.
The sensitivity of silver halide emulsions has been improved by sustained investigation for more than a century. A variety of chemical sensitizations, such as noble metal (e.g., gold), middle chalcogen (e.g., sulfur and/or selenium), and reduction sensitizations, have been developed which, singly and in combination, are capable of improving the sensitivity of silver halide emulsions. When chemical sensitization is extended beyond optimum levels, relatively small increases in speed are accompanied by sharp losses in image discrimination (maximum density minus minimum density) resulting from sharp increases in fog (minimum density). Optimum chemical sensitization is the best balance among speed, image discrimination, and minimum density for a specific photographic application.
Usually the sensitivity of the silver halide emulsions is only negligibly extended beyond their spectral region of intrinsic sensitivity by chemical sensitization. The sensitivity of silver halide emulsions can be extended over the entire visible spectrum and beyond by employing spectral sensitizers, typically methine dyes. Emulsion sensitivity beyond the region of intrinsic sensitivity increases as the concentration of spectral sensitizer increases up to an optimum and generally declines rapidly thereafter. (See Mees, Theory of the Photographic Process, Macmillan, 1942, pp. 1067-1069, for background.)
Within the range of silver halide grain sizes normally encountered in photographic elements the maximum speed obtained at optimum sensitization increases linearly with increasing grain size. The number of absorbed quanta necessary to render a grain developable is substantially independent of grain size, but the density that a given number of grains will produce upon development is directly related to their size. If the aim is to produce a maximum density of 2, for example, fewer grins of 0.4 micron as compared to 0.2 micron in average diameter are required to produce that density. Less radiation is required to render fewer grains developable.
Unfortunately, because the density produced with the larger grains is concentrated at fewer sites, there are greater point-to-point fluctuations in density. The viewer's perception of point-to-point fluctuations in density is termed "graininess". The objective measurement of point-to-point fluctuations in density is termed "granularity". While quantitative measurements of granularity have taken different forms, granularity is most commonly measured as rms (root mean square) granularity, which is defined as the standard deviation of density within a viewing microaperture (e.g., 24 to 48 microns). Once the maximum permissible granularity (also commonly referred to as grain, but not to be confused with silver halide grains) for a specific emulsion layer is identified, the maximum speed which can be realized for that emulsion layer is also effectively limited.
From the foregoing it can be appreciated that over the years intensive investigation in the photographic art has rarely been directed toward obtaining maximum photographic speed in an absolute sense, but, rather, has been directed toward obtaining maximum speed at optimum sensitization while satisfying practical granularity or grain criteria. True improvements in silver halide emulsion sensitivity allow speed to be increased without increasing granularity, granularity to be reduced without decreasing speed, or both speed and granularity to be simultaneously improved. Such sensitivity improvement is commonly and succinctly referred to in the art as improvement in the speed-granularity relationship of an emulsion.
In FIG. 1 a schematic plot of speed versus granularity is shown for five silver halide emulsions 1, 2, 3, 4, and 5 of the same composition, but differing in grain size, each similarly sensitized, identically coated, and identically processed. While the individual emulsions differ in maximum speed and granularity, there is a predictable linear relationship between the emulsions, as indicated by the speed-granularity line A. All emulsions which can be joined along the line A exhibit the same speed-granularity relationship. Emulsions which exhibit true improvements in sensitivity lie above the speed-granularity line A. For example, emulsions 6 and 7, which lie on the common speed-granularity line B, are superior in their speed-granularity relationships to any one of the emulsions 1 through 5. Emulsion 6 exhibits a higher speed than emulsion 1, but no higher granularity. Emulsion 6 exhibits the same speed as emulsion 2, but at a much lower granularity. Emulsion 7 is of higher speed than emulsion 2, but is of a lower granularity than emulsion 3, which is of lower speed than emulsion 7. Emulsion 8, which falls below the speed-granularity line A, exhibits the poorest speed-granularity relationship shown in FIG. 1. Although emulsion 8 exhibits the highest photographic speed of any of the emulsions, its speed is realized only at a disproportionate increase in granularity.
The importance of speed-granularity relationship in photography has led to extensive efforts to quantify and generalize speed-granularity determinations. It is normally a simple matter to compare precisely the speed-granularity relationships of an emulsion series differing by a single characteristic, such as silver halide grain size. The speed-granularity relationships of photographic products which produce similar characteristic curves are often compared. However, universal quantitative speed-granularity comparisons of photographic elements have not been achieved, since speed-granularity comparisons become increasingly judgmental as other photographic characteristics differ. Further, comparisons of speed-granularity relationships of photographic elements which produce silver images (e.g., black-and-white photographic elements) with those which produce dye images (e.g., color and chromogenic photographic elements) involve numerous considerations other than the silver halide grain sensitivites, since the nature and origin of the materials producing density and hence accounting for granularity are much different. For elaboration of granularity measurements in silver and dye imaging attention is directed to "Understanding Graininess and Granularity", Kodak Publication No. F-20, Revised 11-79 (available from Eastman Kodak Company, Rochester, New York 14650); Zwick, "Quantitative Studies of Factors Affecting Granularity", Photographic Science and Engineering, Vol. 9, No. 3, May-June, 1965; Ericson and Marchant, "RMS Granularity of Monodisperse Photographic Emulsions", Photographic Science and Engineering, Vol. 16, No. 4, July-August 1972, pp. 253-257; and Trabka, "A Random-Sphere Model for Dye Clouds", Photographic Science and Engineering, Vol. 21, No. 4, July-August 1977, pp. 183-192.
Silver halide emulsions other than silver bromoiodides find limited use in camera speed photographic elements. A silver bromoiodide emulsion having outstanding silver imaging (black-and-white) speed-granularity properties is illustrated by Illingsworth U.S. Pat. No. 3,320,069, which discloses gelatino-silver bromoiodide emulsions in which the iodide preferably comprises from 1 to 10 mole percent of the halide. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed; e.g., a grain consisting of silver bromoiodide containing 40 mole percent iodide also contains 60 mole percent bromide.) The emulsion is sensitized with a sulfur, selenium, or tellurium sensitizer. The emulsion, when coated on a support at a silver coverage of between 300 and 1000 mg per square foot (0.0929 m.sup.2) and exposed on an intensity scale sensitometer, and processed for 5 minutes in Kodak Developer DK-50.RTM. (an N-methyl-p-aminophenol sulfate-hydroquinone developer) at 20.degree. C. (68.degree. F.), has a log speed of 280-400 and a remainder (resulting from subtracting its granularity value from its log speed) of between 180 and 220. Gold is preferably employed in combination with the sulfur group sensitizer, and thiocyanate may be present during silver halide precipitation or, if desired, may be added to the silver halide at any time prior to washing. (Uses of thiocyanate during silver halide precipitation and sensitization are illustrated by Leermakers U.S. Pat. No. 2,221,805, Nietz et al U.S. Pat. No. 2,222,264, and Damschroder U.S. Pat. No. 2,642,361.) The Illingsworth emulsions also provide outstanding speed-granularity properties in color photography, although quantitative values for dye image granularity are not provided.
In a few instances the highest attainable photographic speeds have been investigated at higher than the normally useful levels of granularity. Farnell, "The Relationship Between Speed and Grain Size", The Journal of Photographic Science, Vol. 17, 1969, pp. 116-125, reports blue-speed investigations of silver bromide and bromoiodide emulsions in the absence of spectral sensitization. The author observed that with grain sizes greater than about 0.5 micron.sup.2 in projected area (0.8 micron in diameter) no further increase in speed with increasing grain size, as expected based on the assumption that the number of absorbed quanta required for developability is independent of grain size, was observed. Actual declines in speed as a function of increasing grain size are reported. Farnell attributes the decline in sensitivity of large grains to their large size in relation to the limited average diffusion distance of photo-generated electrons which are required to produce latent image sites, since it is the proximity of a few atoms of Ag.degree. produced by capture of photo-generated electrons that produces a latent image site.
Tani, "Factors Influencing Photographic Sensitivity", J. Soc. Photogr. Sci. Technol. Japan, Vol. 43, No. 6, 1980, pp. 335-346, is in agreement with Farnell and extends the discussion of reduced sensitivity of larger silver halide grains to additional causes attributable to the presence of spectral sensitizing dye. Tani reports that the sensitivity of spectrally sensitized emulsion is additionally influenced by (1) the relative quantum yield of spectral sensitization, (2) dye desensitization, and (3) light absorption by dyes. Tani notes that the relative quantum yield of spectral sensitization has been observed to be near unity and therefore not likely to be practically improved. Tani notes that light absorption by grains covered by dye molecules is proportional to grain volume when exposed to blue light and to grain surface area when the grain is exposed to minus-blue light. Thus, the magnitude of the increase in minus-blue sensitivity is, in general, smaller than the increase in blue sensitivity when the size of emulsion grains is increased. Attempts to increase light absorption by merely increasing dye coverage does not necessarily result in increased sensitivity, because dye desensitization increases as the amount of dye is increased. Desensitization is attributed to reduced latent image formation rather than reduced photo-generation of electrons. Tani suggests possible improvements in the speed-granularity relationship of larger silver halide grains by preparing core-shell emulsions to avoid desensitization. Internal doping of silver halide grains to allow the use of otherwise desensitizing dye levels is taught by Gilman et al U.S. Pat. No. 3,979,213.
b. Sharpness
While granularity, because of its relationship to speed, is often a focal point of discussion relating to image quality, image sharpness can be addressed independently. Some factors which influence image sharpness, such as lateral diffusion of imaging materials during processing (sometimes termed "image smearing"), are more closely related to imaging and processing materials than the silver halide grains. On the other hand, because of their light scattering properties, silver halide grains themselves primarily affect sharpness during imagewise exposure. It is known in the art that silver halide grains having diameters in the range of from 0.2 to 0.6 micron exhibit maximum scattering of visible light.
Loss of image sharpness resulting from light scattering generally increases with increasing thickness of a silver halide emulsion layer. The reason for this can be appreciated by reference to FIG. 2. If a photon of light 1 is deflected by a silver halide grain at a point 2 by an angle .theta. measured as a declination from its original path and is thereafter absorbed by a second silver halide grain at a point 3 after traversing a thickness t.sup.1 of the emulsion layer, the photographic record of the photon is displaced laterally by a distance x. If, instead of being absorbed within a thickness t.sup.1, the photon traverses a second equal thickness t.sup.2 and is absorbed at a point 4, the photographic record of the photon is displaced laterally by twice the distance x. It is therefore apparent that the greater the thickness displacement of the silver halide grains in a photographic element, the greater the risk of reduction in image sharpness attributable to light scattering. (Although FIG. 2 illustrates the principle in a very simple situation, it is appreciated that in actual practice a photon is typically reflected from several grains before actually being absorbed and statistical methods are required to predict its probable ultimate point of absorption.)
In multicolor photographic elements containing three or more superimposed silver halide emulsion layers an increased risk of reduction in image sharpness can be presented, since the silver halide grains are distributed over at least three layer thicknesses. In some applications thickness displacement of the silver halide grains is further increased by the presence of additional materials that either (1) increase the thicknesses of the emulsion layers themselves--as where dye-image-providing materials, for example, are incorporated in the emulsion layers or (2) form additional layers separating the silver halide emulsion layers, thereby increasing their thickness displacement--as where separate scavenger and dye-image-providing material layers separate adjacent emulsion layers. Further, in multicolor photographic elements there are at least three superimposed layer units, each containing at least one silver halide emulsion layer. Thus, there is a substantial opportunity for loss of image sharpness attributable to scattering. Because of the cumulative scattering of overlying silver halide emulsion layers, the emulsion layers farther removed from the exposing radiation source can exhibit very significant reductions in sharpness.
Zwick U.S. Pat. No. 3,402,046 discusses obtaining crisp, sharp images in a green-sensitive emulsion layer of a multicolor photographic element. The green-sensitive emulsion layer lies beneath a blue-sensitive emulsion layer, and this relationship accounts for a loss in sharpness attributable to the green-sensitive emulsion layer. Zwick reduces light scattering by employing in the overlying blue-sensitive emulsion layer silver halide grains which are at least 0.7 micron, preferably 0.7 to 1.5 microns, in average diameter, which is in agreement with the 0.6 micron diameter referred to above.
c. Blue and minus-blue speed separation
Silver bromide and silver bromoiodide emulsions possess sufficient native sensitivity to the blue portion of the spectrum to record blue radiation without blue spectral sensitization. When these emulsions are employed to record green and/or red (minus blue) light exposures, they are correspondingly spectrally sensitized. In black-and-white and monochromatic (e.g. chromogenic) photography the resulting orthochromatic or panchromatic sensitivity is advantageous.
In multicolor photography, the native sensitivity of silver bromide and silver bromoiodide in emulsions intended to record blue light is advantageous. However, when these silver halides are employed in emulsion layers intended to record exposures in the green or red portion of the spectrum, the native blue sensitivity is an inconvenience, since response to both blue and green light or both blue and red light in the emulsion layers will falsify the hue of the multicolor image sought to be reproduced.
In constructing multicolor photographic elements using silver bromide or silver bromoiodide emulsions the color falsification can be analyzed as two distinct concerns. The first concern is the difference between the blue speed of the green or red recording emulsion layer and its green or red speed. The second concern is the difference between the blue speed of each blue recording emulsion layer and the blue speed of the corresponding green or red recording emulsion layer. Generally in preparing a multicolor photographic element intended to record accurately image colors under daylight exposure conditions (e.g., 5500.degree.K.) the aim is to achieve a difference of about an order of magnitude between the blue speed of each blue recording emulsion layer and the blue speed of the corresponding green or red recording emulsion layer. The art has recognized that such aim speed differences are not realized using silver bromide or silver bromoiodide emulsions unless employed in combination with one or more approaches known to ameliorate color falsification. Even then, full order of magnitude speed differences have not always been realized in product. However, even when such aim speed differences are realized, further increasing the separation between blue and minus blue speeds will further reduce the recording of blue exposures by layers intended to record minus blue exposures.
By far the most common approach to reducing exposure of red and green spectrally sensitized silver bromide and silver bromoiodide emulsion layers to blue light, thereby effectively reducing their blue speed, is to locate these emulsion layers behind a yellow (blue absorbing) filter layer. Both yellow filter dyes and yellow colloidal silver are commonly employed for this purpose. In a common multicolor layer format all of the emulsion layers are silver bromide or bromoiodide. The emulsion layers intended to record green and red exposures are located behind a yellow filter while the emulsion layer or layers intended to record blue light are located in front of the filter layer. (For specific examples refer to U.S. Patent and Trademark Office Class 430, Photographic Chemistry, subclass 507.)
This arrangement has a number of art-recognized disadvantages. While blue light exposure of green and red recording emulsion layers is reduced to tolerable levels, a less than ideal layer order arrangement is imposed by the use of a yellow filter. The green and red emulsion layers receive light that has already passed through both the blue emulsion layer or layers and the yellow filter. This light has been scattered to some extent, and image sharpness can therefore be degraded. Since the blue recording emulsion produces by far the least visually important record, its favored location nearest the source of exposing radiation does not contribute to image sharpness to the degree that would be realized by similar placement of the red or green emulsion layer. Further, the yellow filter is itself imperfect and actually absorbs to a slight extent in the green portion of the spectrum, which results in a loss of green speed. The yellow filter material, particularly where it is yellow colloidal silver, increases materials cost and accelerates required replacement of processing solutions, such as bleaching and bleach-fixing solutions.
Still another disadvantages associated with separating the blue emulsion layer or layers of a photographic element from the red and green emulsion layers by interposing a yellow filter is that the speed of the blue emulsion layer is decreased. This is because the yellow filter layer absorbs blue light passing through the blue emulsion layer or layers that might otherwise be reflected to enhance exposure. One approach for increasing speed is to move the yellow filter layer so that it does not lie immediately below the blue emulsion. This is taught by Lohmann et al U.K. Pat. No. 1,560,963; however, the patent admits that blue speed enhancement is achieved only at the price of impaired color reproduction in the green and red sensitized emulsion layers lying above the yellow filter layer.
A number of approaches have been suggested for eliminating yellow filters, but each has produced its own disadvantages. Gaspar U.S. Pat. No. 2,344,084 teaches locating a green or red spectrally sensitized silver chloride or chlorobromide layer nearest the exposing radiation source, since these silver halides exhibit only negligible native blue sensitivity. Since silver bromide possesses high native blue sensitivity, it does not form the emulsion layer nearest the exposing radiation source, but forms an underlying emulsion layer intended to record blue light.
Mannes et al U.S. Pat. No. 2,388,859 and Knott et al U.S. Pat. No. 2,456,954 teach avoiding blue light contamination of the green and red recording emulsion layers by making these layers 50 or 10 times slower, respectively, then the blue recording emulsion layer. The emulsion layers are overcoated with a yellow filter to obtain a match in sensitivities of the blue, green, and red recording emulsion layers to blue, green, and red light, respectively, and to increase the separation of the blue and minus blue speeds of the minus blue recording emulsion layers.
This approach allows the emulsion layers to be coated in any desired layer order arrangement, but retains the disadvantage of employing a yellow filter as well as additional disadvantages. In order to obtain the sensitivity differences in the blue and minus blue recording emulsion layers without the use of a yellow filter layer to implement the teachings of Mannes et al and Knott et al relatively much larger silver bromide or bromoiodide grains are employed in the blue recording emulsion layer. Attempts to obtain the desired sensitivity differences relying on differences in grain size alone cause the blue emulsion layers to be excessively grainy and/or the grain size of the minus blue recording emulsion layers to be excessively small and therefore of relatively low speed. To ameliorate this difficulty it is known to increase the proportion of iodide in the grains of the blue recording emulsion layer, thereby increasing its blue sensitivity without increasing its grain size. Still, if the minus blue recording emulsion layers are to exhibit more than very moderate photographic speeds, obtaining blue recording emulsion layers of at least 10 times greater speed is not possible within normally acceptable levels of grain, even with increased iodide in the blue recording emulsion layer.
While yellow filters are employed to reduce blue light striking underlying emulsion layers, they by no means eliminate the transmission of blue light. Thus, even when yellow filters are employed, additional benefits can be realized by the further separation of blue and minus blue sensitivities of silver bromide and bromoiodide emulsion layers intended to record in the minus blue portion of the spectrum.
Although silver chloride and chlorobromide emulsions can be put to use as minus blue recording layers in multicolor photographic elements without yellow filter protection, as suggested by Gaspar, cited above, it should be realized that these emulsions also absorb blue radiation, albeit at reduced levels. There are applications where even the small levels of absorption in the blue portion of the spectrum (often referred to as "tail absorption") of these silver chloride-containing emulsions can be disadvantageous. For example, if it is desired to imagewise expose a camera speeds a photographic element having at silver chloride emulsion layer to radiation outside of the blue portion of the spectrum (e.g., green, red, or infrared) and thereafter process the photographic element in the presence of blue light, the emulsion layers can exhibit sufficient native blue sensitivity to increase in background density or fog as a result of work area lighting. Although the blue sensitivity of the chloride-containing emulsion is only a small fraction of its sensitivity to the radiation employed during imagewise exposure, the duration of exposure to process light is much, much longer. Hence even silver chloride and chlorobromide emulsions can benefit by reduction of their blue sensitivity in relation to their sensitivity in another spectral region.
d. Tabular silver halide grains
A variety of regular and irregular grain shapes have been observed in silver halide photographic emulsions. Regular grains are often cubic or octahedral. Grain edges can exhibit rounding due to ripening effects, and in the presence of strong ripening agents, such as ammonia, the grains may even be spherical or near spherical thick platelets, as described, for example by Land U.S. Pat. No. 3,894,871 and Zelikman and Levi Making and Coating Photographic Emulsions, Focal Press, 1964, pp. 221-223. Rods and tabular grains in varied portions have been frequently observed mixed in among other grain shapes, particularly where the pAg (the negative logarithm of silver ion concentration) of the emulsions has been varied during precipitation, as occurs, for example in single-jet precipitations.
Tabular silver bromide grains have been extensively studied, often in macro-sizes having no photographic utility. Tabular grains are herein defined as those having two substantially parallel crystal faces, each of which is substantially larger than any other single crystal face of the grain. The term "substantially parallel" as used herein is intended to include surfaces that appear parallel on direct or indirect visual inspection at 10,000 times magnification. The aspect ratio--that is, the ratio of diameter to thickness--of tabular grains is substantially greater than 1:1. High aspect ratio tabular grain silver bromide emulsions were reported by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125.
From 1937 until the 1950's the Eastman Kodak Company sold a Duplitized.RTM. radiographic film product under the name No-Screen X-Ray Code 5133. The product contained as coatings on opposite major faces of a film support sulfur sensitized silver bromide emulsions. Since the emulsions were intended to be exposed by X-radiation, they were not spectrally sensitized. The tabular grains had an average aspect ratio in the range of from about 5 to 7:1. The tabular grains accounted for greater than 50% of the projected area while nontabular grains accounted for greater than 25% of the projected area. The emulsion having the highest average aspect ratio, chosen from several remakes, had an average tabular grain diameter of 2.5 microns, an average tabular grain thickness of 0.36 micron, and an average aspect ratio of 7:1. In other remakes the emulsions contained thicker, smaller diameter tabular grains which were of lower average aspect ratio.
Although tabular grain silver bromoiodide emulsions are known in the art, none exhibit a high average aspect ratio. A discussion of tabular silver bromoiodide grains appears in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of Silver Iodide Upon the Structure of Bromo-Iodide Precipitation Series", The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and Smith observed a pronounced reduction in both grain size and aspect ratio with the introduction of iodide. Gutoff, "Nucleation and Growth Rates During the Precipitation of Silver Halide Photographic Emulsions", Photographic Sciences and Engineering, Vol. 14, No. 4, July-August 1970, pp. 248-257, reports preparing silver bromide and silver bromoiodide emulsions of the type prepared by single-jet precipitations using a continuous precipitation apparatus.
Bogg, Lewis, and Maternaghan have recently published procedures for preparing emulsions in which a major proportion of the silver halide is present in the form of tabular grains. Bogg U.S. Pat. No. 4,063,951 discloses forming silver halide crystals of tabular habit bounded by {100} cubic faces and having an aspect ratio (based on edge length) of from 1.5 to 7:1. The tabular grains exhibit square and rectangular major surfaces characteristic of {100} crystal faces. Lewis U.S. Pat. No. 4,067,739 discloses the preparation of silver halide emulsions wherein most of the crystals are of the twinned octahedral type by forming seed crystals, causing the seed crystals to increase in size by Ostwald ripening, and completing grain growth without renucleation or Ostwald ripening while controlling pBr (the negative logarithm of bromide ion concentration). Maternaghan U.S. Pat. Nos. 4,150,994, 4,184,877, and 4,184,878, U.K. Pat. No. 1,570,581, and German OLS publications Nos. 2,905,655 and 2,921,077 teach the formation of silver halide grains of flat twinned octahedral configuration by employing seed crystals which are at least 90 mole percent iodide. Lewis and Maternaghan report increased covering power. Maternaghan states that the emulsions are useful in camera films, both black-and-white and color. Bogg specifically reports an upper limit on aspect ratios to 7:1, but, from the very low aspect ratios obtained by the example (2:1), the 7:1 aspect ratio appears unrealistically high. It is clear from repeating examples and viewing the photomicrographs published that the aspect ratios realized by Lewis and Maternaghan were also less than 7:1. Japanese Pat. Kokai No. 142,329, published Nov. 6, 1980, appears to be essentially cumulative with Maternaghan, but is not restricted to the use of silver iodide seed grains.